1. Field of the Invention
The present invention relates to a process for producing an elongated wire material of sintered ceramics; more particularly, it relates to an improvement in a process for producing a wire material which permits the use of a sintered body composed of superconducting compound oxide which exhibits a higher critical temperature of superconductivity.
2. Description of the related art
Under the superconducting condition, the perfect diamagnetism is observed, and no difference in potential is observed so that an electric current of a constant finite value is observed internally, and hence, a variety of applications of superconductivity has been proposed in a field of electric power transmission as a mean for delivering electric power without loss.
The superconductivity can be utilized in the field of power electric applications, such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation, such as magnetic levitation trains or magnetically propelling ships; a highly sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like; in the medical field such as high-energy beam radiation unit; in the field of science, such as NMR or high-energy physics; or in the field of fusion power generation.
In addition to the above-mentioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device, which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, their actual usage has been restricted because the phenomenon or superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure shows rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge, which showed the highest Tc, could not exceed 23.2K at most. This means that liquidized helium (boiling point of 4.2K) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also requires a large-scaled system for liquefaction. Therefore, there has been a strong demand for another superconducting materials having higher Tc. But no materials which exceeded the above-mentioned Tc had been found in studies for the past ten years.
It has been known that certain ceramics material of compound oxides exhibits the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity, and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These type superconductors, however, possess a rather lower transition temperature of about 10k, and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La,Sr].sub.2 CuO.sub.4 which is known as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show is much higher Tc as 30K than the known superconducting materials, and hence it becomes possible to use liquidized hydrogen (b.p.=20.4K) or liquidized neon (b.p.=27.3K) as a cryogen which bring them to exhibit the superconductivity.
It was also reported in the newspaper that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90K in February 1987, and hence possibility of existence of high-temperature superconductors has burst on the scene.
However, the above-mentioned new type of superconducting materials just born has been studied and developed only in a form of sintered bodies as a bulk produced from powders but has not been tried to be shaped into a wire form. The reason is that the new type superconductors are ceramic materials of compound oxide which do not possess enough plasticity or process-ability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy, and, therefore, they cannot or are difficult to be shaped or formed into an elongated article such as a wire by a conventional technique, such as wire-drawing, in which superconducting metal is drawn directly or in embedded condition in copper into a wire form.
It is proposed in Japanese patent laid-open No. 61-131,307 a method for manufacturing a superconducting wire from a metal type superconducting material which is apt to be oxidized and very fragile such as PbMo.sub.0.35 Sg, comprising charging the material metal powder in a metal shell, extruding the metal shell filled with the material powder at a temperature of higher than 1,000.degree. C., and then drawing the extruded composite. This metal working technique, however, cannot apply directly to ceramic material consisting of compound oxide, because the compound oxide type superconducting materials cannot exhibit the superconductivity if the specified or predetermined crystal structure is not realized. In other words, a superconducting wire which shows higher critical temperature and higher critical current density and which is useable in actual applications cannot be obtained outside predetermined optimum conditions. In particular, if the shell is not selected from proper materials, the resulting compound oxide will be reduced due to chemical reaction with the metal of the shell, resulting in poor or inferior properties of superconductivity.
In addition, the above-mentioned sintered ceramic materials must be shaped into an elongated structure when they are used as a superconducting wire. However, the above-mentioned superconducting materials obtained in a form of a sintered body are very fragile and are apt to be broken or cracked even under very weak mechanical stress. And hence, when they are shaped into a wire, special attention must be paid to their handling to insure that they will not be broken.
Further, although a variety of new ceramics such as AlN, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or the like are utilized in many applications because of their superior heat resistance and mechanical properties, it has not been easy to produce a wire of ceramic. It has been the general practice of ceramic molding for manufacturing an elongated article such as wires or rods to add an organic binder to the material powder of ceramic so that a mixture of the powder material and the organic binder is shaped into a rod by means of an extruder or a press machine and then the shaped rod is passed directly or through a trimming or cutting stage to an intermediate sintering stage to remove the organic binder before it is fed to the final sintering stage.
The combination of the above-mentioned press-molding and trimming or cutting operations would result in the loss of great amounts of expensive ceramics material, and a dimensional ratio of longitudinal direction to cross sectional direction of the rod would not be increased. Therefore, this process cannot be used in practice because of waste of material and lower productivity.
The extrusion technique is much better than the press-molding technique in the economy of material and productivity but requires great quantities of organic binder added to the powder material. This organic binder is difficult to be removed completely during the intermediate sintering stage and hence remains in the finally sintered article, resulting in a cause of defects of the product which will lower the strength and the resistance to flexion.
A polycrystal having completely uniform crystal structure cannot be obtained from particles having superconducting properties along. Furthermore, the phenomenon of superconductivity is apt to be easily broken in a stronger magnetic field and under the fluctuation or unhomogeneous distribution of temperature in the sintered body, as well as the above-mentioned oxide type superconducting materials possessing rather higher specific resistance and lower heat-conductivity above the critical temperature. Therefore, if the phenomenon of superconductivity breaks locally, the sintered body produces Joule heat caused by the superconducting current preserved therein, and explosive evaporation of cryogen is induced when the heated portion of the sintered body comes in contact with the cryogen. In order to avert this danger, in conventional metal type superconducting material, superconducting metal is shaped in a form of a fine wire or filament, a plurality of which is embedded in electroconductive metal which plays a role of a by-pass of electric current when superconductivity breaks.
The oxide type superconducting materials are, however, difficult to be shaped or formed into such filaments, because they contain insufficient plasticity or processability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy.
In order to realize a reliable and practical superconducting structure, it is essential that the structure possess enough strength and tenacity to sufficiently to endure bending force during usage and which also has a finer cross sectional dimension in such manner that it can transmit currency at higher critical current density and at higher critical temperature. However, conventional techniques cannot or are difficult to produce wire shaped ceramic articles possessing satisfactory mechanical strength and tenacity as well as a higher dimensional ratio of length to cross section.
Taking the abovementioned situation into consideration, the present inventors have proposed processes for producing sintered ceramic wires having a practically usable higher dimensional ratio of length to cross section without using organic binder, which is a cause of deterioration of strength and tenacity in U.S. patent application Ser. No. 152,713 titled "Process for Manufacturing a Superconducting Wire of Compound Oxide-Type Ceramic" filed Feb. 5, 1988 and Ser. No. 161,480 titled "Process for Manufacturing a Compound Oxide-Type Superconducting Wire" filed Feb. 28, 1988 in which a metal pipe filled with material powder is subjected to plastic deformation such as a wire-drawing technique by means of a die, and then is sintered.
These solutions are in themselves satisfactory but remain such problems that the resulting wire has a tendency to be broken due to relatively lower strength. The present inventors have continued to develop an improved wire-drawing process which can produce a ceramic wire possessing higher strength and no breakage which will complete the present invention.
Therefore, an object of the present invention is to overcome the above-mentioned problems of the conventional technique and to provide an improved process for producing a superconducting wire which has a higher Tc and higher stability as superconductor and which also has a higher degree of freedom in configuration so that it can be applicable to a practical use.